but It still displayed error, even though I had gdal_csharp.dll in my web bin folder:

Please copy the umanaged dll's into your bin folder from javascript:window.location.href='http://www.codeplex.com/SharpMap/Wiki/View.aspx?title=Extensions';.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.Exception: Please copy the umanaged dll's into your bin folder from javascript:window.location.href='http://www.codeplex.com/SharpMap/Wiki/View.aspx?title=Extensions';.
Source Error:
Line 548: {
Line 549: if (ex.GetType() == typeof(TypeInitializationException))
Line 550: throw new Exception(
Line 551: "Please copy the umanaged dll's into your bin folder from javascript:window.location.href='http://www.codeplex.com/SharpMap/Wiki/View.aspx?title=Extensions';.");
Line 552: }
Source File: ....\Trunk\DemoWebSite\App_Code\MapHelper.cs Line: 550
I also went to